What's meant by Average rating to sort out positions? By who or how arrived at?

Each team has a rating (based on the Elo system) which changes every time they play. I simply averaged those individual ratings for each team each year. Any time a team is rated over 2000, it is pretty strong, so to maintain that sort of rating over an entire season gives some measure of their quality.

Only 10% of teams have maintained such a level in the period, and 2% have maintained over 2100.